An MR read transducer requires an active region and passive regions. The active region comprises suitable means for transversely biasing the transducer, and each passive region comprises one of a pair of conductor leads and generally suitable means for longitudinally biasing the transducer.
The most pertinent prior art known to applicants is U.S. Pat. No. 4,771,349. This patent discloses (in FIG. 2) an MR read transducer having passive end regions separated by a central active region. A continuous thin film of MR material is first deposited over both the central region and end regions. A nonmagnetic spacer layer and a soft magnetic bias layer are deposited and are then patterned to cover only the central active region for imparting a transverse bias to the transducer. A longitudinal magnetic bias layer and conductive leads extend over only the end regions of the transducer. This bias layer creates an interfacial exchange interaction with the MR film that results in an effective bias field that is oriented longitudinally for domain suppression.
While this transducer operates satisfactorily, it is relatively difficult to fabricate for the following reason. It is made by applying a mask or "stencil" over the soft magnetic layer and nonmagnetic spacer layer and etching or ion milling to remove these layers outside the mask to define the central region. The problem is that the MR film is generally very thin (e.g., less than 200.ANG.) and it is difficult to stop the etching or milling with a precision that is high with respect to the thickness of the thin MR film. Failure to completely remove the spacer layer in the end regions can render the transducer inoperative; whereas if part of the thin MR film is inadvertently removed, the consequent reduction in its thickness can undesirably impact the sensing characteristics of the transducer. If an attempt is made to restore the MR film to proper thickness by redeposition of MR material, it is difficult to determine how much to deposit because it is difficult to determine in situ how much material was removed. This difficulty in fabrication is also due in part to the fact that the film of MR material is deposited on the substrate as the first step in the fabrication process.
U.S. Pat. No. 5,014,147 (in FIG. 4) discloses an MR transducer in which a soft magnetic bias layer is deposited on a substrate, and covered in turn by a nonmagnetic spacer layer, an MR layer, an antiferromagnetic layer and conductor leads. This patent also discloses (in FIG. 6) an MR transducer in which a longitudinal hard bias layer is deposited upon an auxiliary layer and an MR layer is deposited upon the bias layer, followed by a spacer layer, a soft magnetic bias layer and conductor leads. In these embodiments and also in a variation shown in FIG. 8, all the layers extend the full length of the transducer except for the conductor leads. However, the principal disadvantage of this configuration is that the transducer is longitudinally biased over its entire length, which degrades its transverse bias performance and its signal sensitivity.
U.S. Pat. No. 5,018,037 discloses an MR read transducer in which an MR layer, a spacer layer of nonmagnetic material and a soft magnetic bias layer are formed as full films and then patterned to cover only a central active region of the transducer. Longitudinal bias layers and conductor leads are then formed in end regions only of the transducer. Note (in FIG. 5) the butted junction in the current path between the MR layer and hard magnetic bias layer. Thus the transducer is composite of three abutting sections which can impact the reliability of the transducer and introduce complications in the magnetic and electrical behavior at the junctions of these sections.
Two papers by Tsang, et al. were published in Journal of Applied Physics: (I) in March 1981 at pp. 2471 et seq. and (II) in March 1982 at pages 2605 et seq. Paper (I) discloses use of a NiFe/MnFe/NiFe composite film as an exchange layer in an MR transducer and suggested that such a film would suppress Barkhausen noise. However, it does not disclose or suggest how this exchange layer may be incorporated in an MR read transducer of the type disclosed by applicants. Paper (II) indicated that an exchange structure comprising an MnFe film below (instead of above) a Cu seed layer provided higher exchange magnitude and coercivity at room temperature, higher blocking temperature (T.sub.c), and lower exchange roll off as a function of temperature. However, this paper did not disclose or suggest use of an NiFe film as a seed layer with an MnFe film in an MR read transducer of the type disclosed by applicants.
None of these references discloses or suggests an MR transducer structure in which an MR layer is deposited as a continuous film across the entire width of the transducer during the last step in the fabrication process. This desirably (a) produces an MR transducer with an MR film that provides a continuous current-carrying platform without butted junctions in the current path; and (b) facilitates fabrication by eliminating all etching steps that might require stopping the etching process at a critical interface.